(a) There is no doubt that the texts of the various Berachos were formalized by the Anshei Knesset ha'Gedolah, as is quite clearly recorded in Berachos 33a. The Rambam writes about this in a number of places. For example:

בהל' ברכות פ"א ה"ה, כתב, וז"ל, "ונוסח כל הברכות עזרא ובית דינו תקנום ואין ראוי לשנותם".

ובהל' קריאת שמע פ"א ה"ז כתב, וז"ל, "ברכות אלו עם שאר כל הברכות הערוכות בפי כל ישראל עזרא הסופר ובית דינו תקנום וכו'".

ובהלכות תפילה פ"א ה"ד כתב, וז"ל, "כיון שגלו ישראל בימי נבוכדנצר הרשע וכו' וכיון שראה עזרא ובית דינו כך עמדו ותקנו להם שמונה עשרה ברכות על הסדר וכו' כדי שיהיו ערוכות בפי הכל וכו' ומפני ענין זה תקנו כל הברכות והתפילות המסודרות בפי כל ישראל וכו'", עכ"ל.

(b) However, some Berachos were clearly added later, such as the Berachah of ha'Tov veha'Metiv, which was added at the time of destruction of Beitar (Berachos 48b) and v'Lamalshinim which was added by Shmuel ha'Katan (Berachos 28b).

In addition, some Berachos were adapted and changed over the years, where it became essential, such as Berachos that were affected by the Churban Beis ha'Mikdash. Thus we now say

"המחזיר שכינתו לציון" , whereas originally the Chasimah was "שאותך לבדך ביראה נעבוד" (as we say in Chutz L'Aretz on Yom Tov before Birchas Kohanim, after v'Se'arev; see Rashi Yoma 68b, Yerushalmi Yoma 7:1). This will also explain why Edot ha'Mizrach have a different Nusach of this Berachah than the Ashkenazim. The Edot ha'Mizrach say "תשכון בתוך ירושלים " whereas Ashkenazim changed the text after the Churban to "ולירושלים עירך ברחמים תשוב כו'".

There is also a basic difference between Birchos ha'Nehenin and Birchos ha'Mitzvos on the one hand, and Birchos Shevach v'Hoda'ah and Berachos Al ha'Re'iyah on the other. This is abundantly apparent from the Rambam who writes:

בהלכות ברכות פ"י ה"א, וז"ל, "ברכות אחרות ודברים אחרים הרבה שאין בהן פתיחה ולא חתימה תיקנו חכמים דרך שבח והודיה להקב"ה וכו' " (ע"ש).

It seems that the power to institute these Berachos as necessary still resided with Chazal even after the dissolution of the institution of Anshei Knesset ha'Gedolah. Hence, Rav Ashi came to the same conclusion as Rav Hamnuna of his own accord without any tradition from the Anshei Knesset ha'Gedolah (Berachos 57b) with regard to a particular Birchas ha'Shevach.

However, after the close of the Talmud this power was withdrawn and we do not see any new Berachos introduced after that period. Thus, the fact that the Berachah of "הנותן ליעף כח" is not found in our Gemara has caused some difficulty (see Tur and Bach to OC 46:6 and Aruch ha'Shulchan OC 46:7).

(c) It must be remembered, though, that everything was recited Ba'al Peh at the time of Chazal. Oral law was not permitted to be recorded in writing and in any event there were no mass media, so inevitably there may have been slight differences in Nusach. See, for example, the Gemara about Modim d'Rabanan which concludes הילכך נימרינהו לכולהו"" (Sotah 40a). The same applies regarding various other Berachos, such as the Berachah on the rainbow (Berachos 59a) and on rain (Berachos 59b and Ta'anis 7a) and the Berachos after the Megilah (Megilah 21b), etc. Similarly, in Beitzah 17a Beis Hillel argue with Beis Shammai over the number of Berachos that are recited when Yom Tov falls on Shabbos.

One must also appreciate the ordeals of the long and arduous Galus, especially before the advent of printing. Even after Sidurim were printed the trouble did not stop because a lot of our discrepancies come from the censor. For example, we say "v'Lamalshinim" whereas the original text was "Minim" or "Meshumadim"; "Zedim" replaced "Malchus Zadon," etc. The variant Nuscha'os of the Berachah of שלא עשני גוי, נכרי, עכוםare another example.

There is much more to discuss on this interesting subject but the above general principles should suffice to answer your basic question.

בברכת כל טוב

Yours sincerely,

Rabbi Joseph Pearlman